Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/430

320 while at the opposite point of contact on the S.E. the existing dip might remain comparatively unaffected. The upheaval along this ëline of fault, with which the porphyritic rock (a) is believed to be contemporaneous, probably took place prior to the deposition of the Old Red Sandstone, which, overlying the Lower Silurian rocks unconformably,dips generally seawards in accommodation to what would seem to have been a previously elevated coast-line. Throughout the auriferous district the red granite (b) appears to answer the description given by Prof. Harkness — an intrusive rock "conforming in its course to the strike of the previously elevated strata." The binary compound of felspar and quartz referred to as granitoid (c), and apparently associated with the richest auriferous drifts, seems in many instances to suggest a metamorphosed Lower Silurian rock whose particles, yielding to such agencies as heat and electricity, were melted and mineralized in situ. It does not appear to me altogether inconsistent with this opinion that the rock in question, which is as thinly bedded as the flaggy sedimentary strata, and alternates with these, whether they be gneissose, quartzose, or micaceous, should occur as a short transverse dyke, as it seems to do at Suisgill (fig. 2). If we suppose that fissures caused by local upheaval might have been contemporaneous with an exhibition of metamorphic action, these fissures would be filled by a molten rock, whether its materials were supplied on the spot or intruded from beneath. If, however, it can be shown that rocks are never melted by the agencies which conduce to metamorphism, then both b and c seem to

Fig. 3. — Granite and Trap in Mica-schist, above Crask Bridge, and near Kil-Donnan Lodge.

1. Micaeous Schist, dipping N.E. 2. Granite (b) 3. Trap Dyke.

be as truly plutonic and intrusive as the trap-dyke at Crask Bridge on the Ullie (fig. 3).

4. Referring to the accompanying map of the auriferous district, (Plate XIII.) I beg to offer a few detailed descriptions, beginning on the S.W. at

The Uisge-dubh, or Blackwater, runs nearly across the strike of highly inclined flaggy quartzites and micaceous schists, whose dip ranges from S. to E. At the waterfall, below Ceann-an-tuir, granite (b) appears among beds of micaceous schist which have been dislocated by a fault. Parallel courses of this rock recur at short